AVR bit manipulation compilation in GCC: can not get SEI instead of read-modify-write

Question

I'm trying to set bit 2 in TIMSK register and using the following C code:

TIMSK |= (1<<2);

Compiler makes the following code:

TIMSK |= (1<<TOIE1);
108: 89 b7 in r24, 0x39 ; 57
10a: 84 60 ori r24, 0x04 ; 4
10c: 89 bf out 0x39, r24 ; 57

So it is Read-Modify-Write operation which is a) slow, b) not very consistent as sometimes it is better not to disturb any excess bits.

However if I'd replace TIMSK with let's say PORTA I will get SEI as expected:

PORTA |= (1<<2);
108: da 9a sbi 0x1b, 2 ; 27

So how do I force the compiler to compile the instruction above as bit instruction instead of RMW?

Or this (TIMSK) register is not bit accessible?

• Asmyldof: The resulting code is not disturbing any excess bits. It reads the register, performs the OR and writes the result, it's exactly the OR-IS you typed in C code. Further, SEI is the global interrupt enable instruction (brother of CLI), you mean SBI as in your second code block.

Answer 1

I found it:

Citation from original datasheet (page 20):

I/O Registers within the address range 0x00 - 0x1F are directly bit-accessible using the SBI and CBI instructions

So only first 32 registers are bit-accessible. TIMSK has address 0x39 - so it's pretty high above the range :(

• Asmyldof: Exactly. It's a trade-off for the choices they made way back when to keep things stream-lined in most-use situations. They do try to get the most likely candidates into the lower 32, though. How often do you write TIMSK? How often PortA/B/C?